Polyester has been used in a wide variety of applications including fibers, films, and bottles because of its excellent properties. For example, polyethylene terephthalate, which is obtained by polycondensation of ethylene glycol and terephthalic acid, has been used in many applications because of its excellent mechanical strength, chemical properties, and the like, and mass-produced around the world as a synthetic fiber most suitable for clothes. For example, for polytrimethylene terephthalate which is made from 1,3-propanediol and terephthalic acid, its market is expanding because inexpensive 1,3-propanediol synthesis methods have been developed in recent years, and polytrimethylene terephthalate is expected to be used for clothes of soft texture making use of polymer properties such as excellent elastic recovery after elongation and a low Young's modulus.
In more recent years, biomass resource-derived polyesters have received attention out of concern about steep price increase and depletion of petroleum resources. In addition to polylactic acid obtained by polymerizing lactic acid obtained by a fermentation method, polybutylene succinate obtained by synthesizing a monomer through chemical conversion of succinic acid obtained by a fermentation method and polymerizing the monomer has been developed as a biomass resource-derived polyester (Japanese Patent No. 4380654). Biomass resource-derived polyester materials have a problem in that biomass resource-derived impurities may be contained, and as a means for solving the problem, methods of removing biomass resource-derived impurities using a nanofiltration membrane have been developed (JP 2007-502325 W and JP 2010-150248 A).
Polyester is formed into various molded articles by molding, and it is known that polyester degradation occurs during its melt molding, which degradation is generally expressed as a thermal weight loss rate. A high thermal weight loss rate can cause deterioration in mechanical properties of resin or reduce molding stability.
Polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate having an aromatic backbone have been developed as a polyester having a low thermal weight loss rate, and further reducing the thermal weight loss rate of these polyesters further enhances their physical properties and, in addition, enables high-speed molding at high temperature.
Thus, it could be helpful to provide a method of producing a polyester having a lower thermal weight loss rate and more excellent mechanical properties and molding stability than those of conventional polyesters.